Journal of Composites for Construction

Previous experimental studies, conducted by some of the authors, on in-plane response of tuff masonry walls strengthened with an innovative cementitious matrix composite grid (CMG) system confirmed that the CMG system could satisfy basic design requirements such as compatibility with the tuff masonry support (i.e., in terms of good bond properties), reversibility of the intervention and strengthening effectiveness. However, very large scatter was found in the experimental outcomes. Micromodeling and some parametric analyses were adopted to understand the contribution of basic material (mortar, tuff blocks and CMG strengthening) and the effect of the workmanship defects on the structural behavior of a natural stone wall. In order to conduct the analyses, finite-element method models of the elements have been compared to experimental data and they were found to be in good agreement with the test data. Significant improvements of strength and in the postpeak response were achieved installing different layouts of the CMG system. However the strengthening intervention had a negligible influence on the initial stiffness of the walls and this means that it has a reduced impact on the behavior of the existing structure.

Research reported herein investigates the out-of-plane impact resistance of unreinforced masonry (URM) walls strengthened with carbon fiber-reinforced polymer (CFRP) composites, externally applied in sheets to one face of the wall. Two analytical methods based on energy principle and wave propagation theory and a finite-element-based numerical model have been developed, assuming a perfect bond at composite–masonry interface with an equivalent stiffness of the system. Full-scale impact tests are conducted for verification purpose, where three $1.2\mathrm{m}$ tall URM concrete walls (one unstrengthened and two strengthened with continuous unidirectional and woven CFRP sheets) are vertically tested up to cracking using a pendulum drop-weight impact tester. The test results compare reasonably well with those obtained from the analyses and simulation. It is found that the energy and finite-element methods can provide reasonable estimates for peak impact force and wall deflection, whereas the wave propagation method is rather limited by its applicability. Parametric studies are conducted to examine the effect of impactor mass, velocity, amount of CFRP reinforcement, and property of masonry material using the developed models.

Hollow core reinforced concrete columns are generally preferred in use to decrease the cost and weight/stiffnesss ratio of members, such as bridge columns and piles. With a simplified stress state assumption, strengthening a hollow core reinforced concrete column with fiber-reinforced polymer (FRP) wrapping provides a biaxial confinement to the concrete, which leads to a need of defining the effect of FRP wrapping on the strength and ductility of the hollow core reinforced concrete columns. In this study, two groups of four hollow core reinforced concrete columns ( $205\mathrm{mm}$ outer diameter, $56\mathrm{mm}$ hollow core diameter, and $925\mathrm{mm}$ height) were tested under concentric, eccentric (25 and $50\mathrm{mm}$ eccentricity) and bending loads to observe the effect of carbon FRP (CFRP) wrapping. All the columns had internal steel reinforcement. Half of the columns had three layers of circumferential CFRP wrapping, whereas the other half had no external confinement. Axial load-bending moment $\left(P\text{–}M\right)$ diagrams of each group were drawn using the obtained experimental results for both groups. It was observed that, CFRP wrapped columns had higher load and moment carrying capacities than the other group. An analytical model is proposed for drawing the $P\text{–}M$ diagram of CFRP wrapped hollow core reinforced concrete columns.

This paper presents the results of a recent study conducted to refine the design-oriented stress–strain model originally proposed by Lam and Teng for fiber-reinforced polymer (FRP)-confined concrete under axial compression. More accurate expressions for the ultimate axial strain and the compressive strength are proposed for use in this model. These new expressions are based on results from recent tests conducted by the writers’ group under well-defined conditions and on results from a parametric study using an accurate analysis-oriented stress–strain model for FRP-confined concrete. They allow the effects of confinement stiffness and the jacket strain capacity to be separately reflected and accounts for the effect of confinement stiffness explicitly instead of having it reflected only through the confinement ratio. The new expressions can be easily incorporated into Lam and Teng’s model for more accurate predictions. Based on these new expressions, two modified versions of Lam and Teng’s model are presented. The first version involves only the updating of the ultimate axial strain and compressive strength equations. The second version caters to stress–strain curves with a descending branch, which is not covered by the original model.

To assess the viability of the external confinement of normal-strength concrete beam–column joints with carbon fiber-reinforced polymer (CFRP) sheets in increasing the bond strength of hooked bars anchored in the joints, 12 hooked bar specimens were tested. The variables were beam tensile bar size, anchorage length, mode of confinement of the beam hooked bars in the beam–column joint (whether the hooked bars were anchored within or outside the column reinforcement cage, denoted as “confined specimens” or “unconfined specimens”), and presence or absence of FRP wraps. The specimen simulated the rigid connection of a cantilever beam to a column. The tensile beam reinforcement consisted of two bars anchored in the base column using hooked-bar anchorages. Test results indicated that FRP sheets were effective in increasing the anchorage capacity and the ductility of the load–deflection history for both unconfined and confined specimens. However, FRP sheets had a more significant influence on unconfined specimens than companion confined specimens. As compared with unconfined specimens without FRP wrapping, unconfined FRP specimens had an average of a 23% increase in bond strength, confined non-FRP specimens had an average 30% increase in bond strength, and confined FRP specimens had an increase of 54%.

A retrofitting technique has been developed that uses carbon fiber-reinforced polymer (CFRP) straps to increase the shear capacity of reinforced concrete beams. The vertical straps are not bonded to the beam but are instead anchored against the beam, which makes this technique potentially more effective than bonded FRP retrofitting techniques. However, it also means that models for bonded FRPs are not appropriate for use with the straps. Instead, a model based on a shear friction approach has been developed where the strain in the straps is calculated based on a term that accounts for the effects of prestress and additional strain in the strap due to shear crack opening. The model can either consider the shear reinforcement to be smeared along the length of the beam or discrete elements. The “smeared” model was checked against an experimental database consisting of rectangular, T-, and deep beams, both in terms of predicted capacity and predicted strain in the straps. Overall the smeared model predicted the capacity of the specimens and, with some adjustments, the strains quite accurately. There were, however, cases when it was more appropriate to use the “discrete” model such as when the transverse reinforcement ratio was low or when the transverse reinforcement spacing was high. Further experimental data are required to fully validate the models and to determine appropriate limits on the use of the smeared model and the discrete model. However, the initial results are promising.

In reinforced concrete (RC) beams strengthened in shear with fiber-reinforced polymer (FRP), crushing of the web can be a potential mode of failure. The guidelines provided by codes and standards for the design of structures strengthened with externally bonded FRP recommend limiting the maximum shear strength to avoid such an undesirable failure scenario. However, these limitation provisions are not based on specific research studies performed on beams strengthened in shear with FRP. Rather, they simply duplicate provisions used in conventional concrete codes and standards. The main objective of this research study is to assess the suitability of the limits specified by the guidelines, and propose, if necessary, an alternative equation as an upper limit for shear strength against web crushing failure in such structures. To this end, an analytical approach was developed based on the static theorem of the theory of plasticity. The predictions of the equations resulting from this approach were compared with those obtained from tests reported in the literature and with those predicted by ACI Committee 440-02, Canadian Standard S6-06, and the European recommendations fib TG 9.3. The study shows that the current ACI Committee 440-02 and Canadian Standards provisions are overly conservative and therefore need to be reviewed.

This paper presents the results of an experimental work involving pultruded beams. The tests developed attempt to observe the interaction between local and global buckling in open-section beams. A modified three-point bending test with both ends clamped has been used in order to reduce the slenderness of the structural elements. By means of a finite-element model the critical bending moment has been calculated. Special care has been taken to obtain an accurate correspondence between the real test and the finite-element model. The comparison made between test results and critical bending moments showed that the above-mentioned interaction clearly reduces the lateral buckling load in the low slenderness range. Based on the experimental data, Dutheil’s formulation has been adjusted leading to a new design equation. The proposed equation showed a good correlation in the low slenderness range, but did not match well with experimental data from literature developed in the high slenderness range. For high slenderness values, using the critical bending moment seems to be the best design method. Therefore, more experimental work has to be done on pultruded beams in order to establish a suitable formulation to describe the transition between the low and high slenderness ranges’ behaviors.

This paper discusses the development of a novel test fixture for applying a uniform radial stress to cylindrical composite laminates in such a way as to eliminate bending moments and axial loads. This fixture requires the use of easily fabricated poly-tetrafluoroethylene (PTFE) ring seals. These seals, which fit just inside the composite sample, are designed to be internally pressurized in the fixture and expand outwards, applying a uniform radial load to the sample. The results of mechanics analyses of the PTFE ring seal and composite sample are presented, which estimate the radial stress applied to the composite sample as a fraction of the applied internal hydraulic pressure. Further, results are presented from a prolonged hygrothermal aging study, in which the E-glass/epoxy, cylindrically wound composite samples are aged under varying hygrothermal conditions (ambient dry; and 30, 40, and $50°\mathrm{C}$ submerged in water). At various points during the aging, a subset of samples are removed and destructively tested to measure the reduction in strength from the initial values. Annular specimens are tested using the fixture, and hoop tensile strengths are recorded. In addition, samples cut axially from the cylindrical composite tubes are used to evaluate tensile and compressive axial strengths. Arrhenius fits are used to estimate activation energies for the strength loss in the axial (compression) and hoop (tension) directions.

We examine here the replacement of a deteriorated concrete deck in the historic Hawthorne Street Bridge in Covington, Va. with a lightweight fiber-reinforced polymer (FRP) deck system (adhesively bonded pultruded tube and plate assembly) to increase the load rating of the bridge. To explore construction feasibility, serviceability, and durability of the proposed deck system, a two-bay section (9.45 by $6.7\mathrm{m}$ ) of the bridge has been constructed and tested under different probable loading scenarios. Experimental results show that the response of the deck is linear elastic with no evidence of deterioration at service load level (HS-20). From global behavior of the bridge superstructure (experimental data and finite- element analysis), degree of composite action, and load distribution factors are determined. The lowest failure load ( $93.6\text{kips}$ or $418.1\mathrm{kN}$ ) is about 4.5 times the design load ( $21.3\text{kips}$ or $94\mathrm{kN}$ ), including dynamic allowance at HS-20. The failure mode is consistent in all loading conditions and observed to be localized under the loading patch at the top plate and top flange of the tube. In addition to global performance, local deformation behavior is also investigated using finite-element simulation. Local analysis suggests that local effects are significant and should be incorporated in design criteria. Based on parametric studies on geometric (thickness of deck components) and material variables (the degree of orthotropy in pultruded tube), a proposed framework for the sizing and material selection of cellular FRP decks is presented for future development of design guidelines for composite deck structures.